In holographic data storage digital data are stored by encoding the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object-beam’, is modulated by a spatial light modulator (SLM) and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recoding. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a single or few two-dimensional layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light intensity variations, i.e. a two-dimensional binary array or an array of grey values, which code multiple bits. Data pages consisting of patterns showing different phases can also be used. This allows achieving increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the SLM and detected with a detector array.
Data pages include synchronization marks, also referred to as sync marks, to determine the exact scaling factor from the SLM to the detector and to correct image distortion. Sync marks usually consist of a specific bit pattern, which is known and can be identified clearly by the reading apparatus. For any holographic data storage system the correct sync mark detection is essential for a successful demodulation procedure. As the scaling factor and the image distortion can vary locally, sync marks are usually distributed over the entire data page. If the sync mark detection fails in a part of the data page then in most cases the demodulation will also fail in this region. Due to defects in the holographic material or distortions such as detector noise the correct detection of a local sync mark may fail.